Walkthrough: After talking with Hau, if you choose to pursue the path to The Battle Tree, you will need to head back to Poni Island. Go through the Poni Wilds to the Ancient Poni Path towards Poni Grove, the area that was originally blocked before this Time. But right before you hit Poni Grove, while still on the Ancient Poni Path, you will be stopped and approached by Dexio and Sina, Professor Sycamore's Assistants. Dexio will then challenge you to a Rematch to once again test your strengths, this time with a Mega Evolution.
Pokemon Trainer Dexio = Level 61 Espeon, Level 61 Raichu, Level 61 Slowking, Level 61 Metagross, Level 61 Alakazam w/Alakazite
After defeating Dexio, he and Sina will explain the process of Mega Evolutions to you. He will then give you a Key Stone, equipping your Z-Ring to Mega Evolve for yourself, as well as your very own Alakazite. After they depart, you will be able to continue your journey to The Battle Tree, starting with Poni Grove.